Analysis of a pulsating fatigue process on carbon steel with different carbide shapes

In the pulsating fatigue process, the relationship between cyclic softening (hardening) and ratcheting should be clarified from the perspective of the practical use of the mechanical parts. However, most previous reports have been limited to predicting of ratcheting curves using constitutive equations. Consequently, there are few qualitative discussions regarding the relationship between cyclic softening (hardening) and ratcheting for changes in the number of cycles. In this study, the plastic strain amplitude, which represents the fatigue damage, is plotted on the X axis and the ratcheting strain rate is plotted on the Y axis, cycle by cycle, to investigate the fatigue and ratcheting damage simultaneously. This figure is known as the SH curve after Dr. Susumu Horibe. As an example, the pulsating fatigue processes of JIS S25C (AISI 1025) with three different carbide shape types are analyzed under the conditions that the engineering stress amplitudes are over their lower yield points. Using SH curves it is shown that regardless of the shape of the carbide, the fatigue behavior should be divided into five stages. Stage I corresponds to the un-pinning of dislocations from the Cottrell atmosphere. Stage II corresponds to the propagation of the Luders band. Stage III corresponds to an increase in short range dislocation movement during the formation of cellular structures due to multiple slip locations. Stage IV corresponds to the fracture of the cellular structure due to an increase in the true stress. Stage V corresponds to crack initiation and propagation. The plastic strain amplitude increases with the number of cycles, while the ratcheting strain rate decreases, especially in stage III; this phenomenon has never been reported previously. At this stage, the microstructure was observed by TEM. It is also determined that in this case, the maximum stress is over the lower yield point, and the ratcheting strain rate is dominant over the fatigue life.